Several systems are known to deliver printing data to the printing engine of a digital printing system. A stringent requirement is that the system has to apply a method capable of delivering data at the speed of the printing system.
One such method is described in the international patent application WO-A-99/24933. This document relates to the merging of compressed raster images in a printing system capable of printing pages containing variable information with unrestricted variability from page to page. Each page is composed of several page elements which are processed by the raster image processor (RIP) in advance and which are stored in a compressed format in a page element cache. These page elements are merged into one page according to the page layout script data. This merging is done while the page elements remain mainly in compressed format. The compressed raster image of the complete page is then delivered to the printer where it is processed by a decompression and screening system which delivers data to the printing device. The method described in WO-A-99/24933 has however certain drawbacks. The continuous tone (“CT”) data is compressed using a block based compression method (e.g. JPEG 8×8, a compression standard of the Joint Photographic Experts Group). In order to make rapid merging of two continuous tone page elements possible, the merging has to occur along CT coding block boundaries. Thus the placing of these blocks has to meet certain criteria or the blocks of one of the page elements are translated in order to obtain an exact overlap of the blocks of the two continuous tone page elements. Such an adjustment can be done while keeping exact registering of the boundaries of the different page elements because the locations of the boundaries are stored independently of the image content of the page elements, but the image of the page element is also translated which can cause problems when the images of two continuous tone page elements need to be in exact registration. In a 300 pixel per inch system (12 pixels/mm) using 8×8 JPEG coding this can lead to shifts of ⅓ mm which can give rise to visible distortions when printing certain images. The merging in compressed format also requires that the page elements need to be compressed using just one compression algorithm. This means that only one line-work data compression format and only one CT data compression format can be used. For instance, merging of page elements which are compressed e.g. using different JPEG formats is not possible in compressed format. Moreover, even when using a single algorithm, the boundary blocks of the CT page elements have to be decompressed, merged and compressed again. When merging elements by superposition of blocks having transparent elements, the blocks have to be decompressed before merging. This implies the need for high processing power. The image information of these blocks is compressed twice, leading to extra loss of image quality.
Also other drawbacks of the known methods exist. It is difficult to merge two different page elements having a different resolution. When a single page element is required at two different locations and orientations on the same page, enough memory space has to be available to store the different copies of the page elements in e.g. different orientation.